(1) Field of the Invention
The present invention relates to a process for preparing shaped articles such as fibers, films and tapes, which do not show a substantial loss of clarity and have an excellent gloss, from wholly aromatic polyamides having a good heat resistance.
(2) Description of the Prior Art
It is known that wholly aromatic polyamides synthesized from aromatic diamines and aromatic dicarboxylic acids are excellent in heat resistance and flame resistance. It also is known that these wholly aromatic polyamides are soluble in certain amide solvents, and shaped articles, such as fibers, can be prepared from these wholly aromatic polyamide solutions in amide solvents according to a dry method, a wet method or a dry jet wet method.
However, the dry method is objectionable in that a large quantity of heat is necessary for removal of the amide solvent, which ordinarily has a high boiling point, by evaporation and the equipment investment is therefore large. Furthermore, wet washing is required for removal of the solvent left in the obtained fibers or for removal of the salt added as a solubilizing aid. Therefore, the dry method is disadvantageous from the economical viewpoint as compared with the wet method.
Various coagulating baths have heretofore been proposed in connection with the wet method. For example, Japanese Patent Publication No. 370/63 discloses a process in which a wholly aromatic polyamide is dissolved in an N,N-dialkylamide and shaped articles, such as fibers or films, are prepared by extruding the so formed solution into an aqueous coagulating bath containing calcium thiocyanate. However, loss of clarity cannot be avoided in shaped articles obtained according to this process and desirable shaped articles cannot be prepared according to this process. Furthermore, Japanese Patent Publications No. 41,743/72 and 17,551/73 proposed the use of a coagulating bath consisting of an aqueous solution of calcium chloride. When this coagulating bath is used, a relatively good homogeneous shaped article exhibiting a reduced loss of clarity can be obtained, but the process using this coagulating bath is still not satisfactory in that when the spinning speed is increased, physical properties of the fibers deteriorate, uneven fibers are often formed, and the spinning operation becomes difficult because of yarn breakage. Therefore, it is difficult to raise the productivity.
Japanese Patent Publication No. 815/67 proposes a process in which a shaped article is obtained by subjecting a solution of a wholly aromatic polyamide in an amide solvent to a dry jet wet shaping procedure using an aqueous coagulating bath consisting of water alone or a combination of water with a small amount of a solvent. In this process, however, loss of clarity of the shaped article in the coagulating bath cannot be avoided, and especially in the case of wholly aromatic polyamides having a high content of m-phenylene groups, for example, poly-m-phenylene-isophthalamide, a copolymer composed mainly of poly-m-phenylene-isophthalamide, poly-m-phenylene-terephthalamide and a copolymer composed mainly of poly-m-phenylene-terephthalamide, loss of clarity in the coagulating bath is conspicuous and it is very difficult to obtain good products. As a means for overcoming this defect, Japanese Patent Publication No. 815/67 proposes a method in which the shaped article, withdrawn from the coagulating bath, is treated with an alcohol, such as ethanol or isopropanol, or an aqueous solution of dimethylacetamide, urea or the like. However, the loss of clarity is not completely avoided, and the use of such a treating liquid makes it complicated and difficult to control the solvent vapor at the step of recovering the solvent.